When in the idling state, an internal combustion engine that functions as a drive unit of a vehicle does not serve to drive the vehicle. Instead, it need only perform the work required to drive its own rotation and, possibly, the additional work required to drive mechanical or electrical auxiliary units (e.g., pumps, servo drives and/or a generator) which may be present and in an "on" state.
To keep fuel consumption, pollutant emissions and noise development low, the idling speed of the engine is generally set as low as possible. For reasons of comfort, however, the idling speed can only be reduced to a certain minimum speed. In the prior art, an idling speed control system maintains the idling speed of the engine as constant as possible at a value just above this minimum speed.
The minimum speed is temperature-dependent. In the cold state of the internal combustion engine (e.g., at start-up), the minimum speed lies higher than it does at, for example, the ordinary operating temperature. This change in the minimum speed is particularly pronounced at low, sub-freezing temperatures (for example, minus 20.degree. C.). To address this temperature dependency, some prior art idle control systems ensure an acceptable idling speed is achieved at all temperatures by using the same average target idling speed at all temperatures of the engine, but uniformly raising the average target idling speed of the engine so that it lies just above the minimum idling speed required at the lowest expected temperature. This approach is disadvantageous, however, because the average target idling speed (and, thus, the actual idling speed) is needlessly high during all operating states except during the short starting phases at the lowest expected temperature.
Some prior art systems address this problem by controlling the idling speed reference value (i.e., the average target idling speed) as a function of temperature such that the idling speed of the internal combustion engine is raised at low temperatures. Prior art systems initially performed this idling speed adjustment through manual control, but for some time now, automatic idling speed control employing a temperature-dependent idling speed reference value has been commonly performed (see, for example, Automotive Handbook/Bosch, 21st Edition, 1991, page 435).
The above noted solution, which has been common for decades, functions, in principle, and is considered the optimal solution with respect to fuel consumption, pollutant emissions and noise development, given the close positioning of the idling speed reference value to the minimum speed at all temperatures that occur in practice.
An example of active damping of torque irregularities by means of an electric machine is known from DE 44 23 557 A1 (there a sector machine).